By Heather Rock Woods
Like Gold Rush sluice boxes that separate gold from fool’s
gold, statistics lets real discoveries glitter instead of masquerading
Statistics played an important role in CERN’s decision
three years ago to turn off the LEP machine and begin on-time construction
of a new machine. Even though there were tantalizing hints of the still
unearthed Higgs particle detected just before LEP was to turn off,
"statistically, there weren’t enough events and the confidence limits were
not as strong as needed to claim a discovery," said Louis Lyons, a
particle physicist at Oxford. CERN decided not to make the expensive
changes to its construction contracts that would be required to keep the
In order to add advanced statistics techniques to the
toolbox of particle physicists, astrophysicists and cosmologists, some of
the world’s greatest statisticians came to SLAC from September 8 to 11 for
the PHYSTAT 2003 conference.
About 120 participants studied advanced statistics for
measurements and searches in their fields, hoping to improve results and
save time and frustration when analyzing the mounds of data accumulated
from an experiment.
"We spend a lot of time, effort and money to build, design
and run apparatus. Getting the most out of your data is very important and
relatively cheap," said conference organizer Lyons. He is comfortable
wielding statistical tools, and has written a book and given many lectures
on the topic.
"Even particle physicists can find statistics a chore," he
said, "but it’s an essential part of correctly understanding what an
experiment has measured and to what accuracy." Statistics is also used to
set a limit on probabilities, check if the data is consistent with
Standard Model predictions, and combine results from different experiments
to create a more sensitive answer.
"We learn statistics the hard way, by trying it out. The
conference really was meant to enhance the statistical ability of people
in these fields," said Lyons. "In particle physics, astrophysics and
cosmology, people work with different tools—accelerators versus
telescopes—but nonetheless a lot of the data analysis techniques are very
Broadening the Audience
Compared to previous physics conferences on statistics,
the SLAC event broadened its audience to the astrophysics and cosmology
community, and invited more statisticians to provide expert insight.
Stanford Statistics Professor Brad Efron gave the keynote
address, entitled "Bayesians, Frequentists and Physicists," about
different approaches to statistics. Efron is also president of the
American Statistical Association and a MacArthur Prize winner.
Seth Digel (GLAST) and Frank Porter (BaBar) gave talks on
statistical issues they face. Jerry Friedman (SCS), a particle physicist
turned professor of statistics at Stanford, spoke on "Modern Developments
in Machine Learning."
The Importance of Limits
Setting limits is an important statistical tool. Many
experiments look for things but don’t see them, like the search for dark
matter and the Higgs particle.
"But rather than say you don’t see it, you can say the
maximum effect that could be there is x," said Lyons. It’s similar to
learning an item is smaller than a breadbox when playing 20 Questions. "If
you set a good limit, it can be very significant."
For example, though LEP did not find the Higgs particle,
its search was very sensitive. Physicists can now say the Higgs particle,
if it exists, has a mass heavier than 114 GeV. LEP’s successor (LHC),
Fermilab’s Tevatron, and any future Linear Collider will have shots at
finding the particle at those greater energies.
Other conference topics included signal significance,
systematics, spatial data, non-parametric estimation, unfolding
convolution, blind analyses, multivariate classification, variability of
sources, hypothesis testing, goodness of fit and cluster analysis.
The local conference committee consisted of Richard Mount,
Arla LeCount, Joseph Perl and David Lee. Local members of the scientific
committee were Roger Barlow, Seth Digel, Brad Efron, Jerry Friedman,
Jeffrey Scargle and Steve Yellin.
For more information, and a recommended reading list, see: